Something strange is afoot in the galaxy known as 1ES 1927+654: In late
2017, and for reasons that scientists couldn't explain, the supermassive
black hole sitting at the heart of this galaxy underwent a massive identity
crisis. Over a span of months, the already-bright object, which is so
luminous that it belongs to a class of black holes known as active galactic
nuclei (AGN), suddenly grew a lot brighter—glowing nearly 100 times more
than normal in visible light.
Now, an international team of astrophysicists, including scientists from CU
Boulder, may have pinpointed the cause of that shift. The magnetic field
lines threading through the black hole appear to have flipped upside down,
causing a rapid but short-lived change in the object's properties. It was as
if compasses on Earth suddenly started pointing south instead of north.
The findings, published May 5 in The Astrophysical Journal, could change how
scientists look at supermassive black holes, said study coauthor Nicolas
Scepi.
"Normally, we would expect black holes to evolve over millions of years,"
said Scepi, a postdoctoral researcher at JILA, a joint research institute
between CU Boulder and the National Institute of Standards and Technology
(NIST). "But these objects, which we call changing-look AGNs, evolve over
very short time scales. Their magnetic fields may be key to understanding
this rapid evolution."
Scepi, alongside JILA Fellows Mitchell Begelman and Jason Dexter, first
theorized that such a magnetic flip-flop could be possible in 2021.
The new study supports the idea. In it, a team led by Sibasish Laha of
NASA's Goddard Space Flight Center collected the most comprehensive data yet
on this far-away object. The group drew on observations from seven telescope
arrays on the ground and in space, tracing the flow of radiation from 1ES
1927+654 as the AGN blazed bright then dimmed back down.
The observations suggest that the magnetic fields of supermassive black
holes may be a lot more dynamic than scientists once believed. And, Begelman
noted, this AGN probably isn't alone.
"If we saw this in one case, we'll definitely see it again," said Begelman,
professor in the Department of Astrophysical and Planetary Sciences (APS).
"Now we know what to look for."
An unusual black hole
Begelman explained that AGNs are borne out of some of the most extreme
physics in the known universe.
These monsters arise when supermassive black holes begin to pull in huge
amounts of gas from the galaxies around them. Like water circling a drain,
that material will spin faster and faster the closer it gets to the black
hole—forming a bright "accretion disk" that generates intense and varied
radiation that scientists can view from billions of light-years away.
Those accretion disks also give rise to a curious feature: They generate
strong magnetic fields that wrap around the central black hole and, like
Earth's own magnetic field, point in a distinct direction, such as north or
south.
"There's increasingly evidence from the Event Horizon Telescope and other
observations that magnetic fields might play a key role in influencing how
gas falls onto black holes," said Dexter, assistant professor in APS.
Which could also influence how bright an AGN, like the one at the heart of
1ES 1927+654, looks through telescopes.
By May 2018, this object's surge in energy had reached a peak, ejecting more
visible light but also many times more ultraviolet radiation than usual.
Around the same time, the AGN's emissions of X-ray radiation began to dim.
"Normally, if the ultraviolet rises, your X-rays will also rise," Scepi
said. "But here, the ultraviolet rose, while the X-ray decreased by a lot.
That's very unusual."
Turning on its head
Researchers at JILA proposed a possible answer for that unusual behavior in
a paper published last year.
Begelman explained that these features are constantly pulling in gas from
outside space, and some of that gas also carries magnetic fields. If the AGN
pulls in magnetic fields that point in an opposite direction to its own—they
point south, say, instead north—then its own field will weaken. It's a bit
like how a tug-of-war team tugging on a rope in one direction can nullify
the efforts of their opponents pulling the other way.
With this AGN, the JILA team theorized, the black hole's magnetic field got
so weak that it flipped upside down.
"You're basically wiping out the magnetic field entirely," Begelman said.
In the new study, researchers led by NASA set out to collect as many
observations as they could of 1ES 1927+654.
The disconnect between ultraviolet and X-ray radiation turned out to be the
smoking gun. Astrophysicists suspect that a weakening magnetic field would
cause just such a change in the physics of an AGN—shifting the black hole's
accretion disk so that it ejected more ultraviolet and visible light and,
paradoxically, less X-ray radiation. No other theory could explain what the
researchers were seeing.
The AGN itself quieted down and returned to normal by summer 2021. But Scepi
and Begelman view the event as a natural experiment—a way of probing close
to the black hole to learn more about how these objects fuel bright beams of
radiation. That information, in turn, may help scientists know exactly what
kinds of signals they should look for to find more weird AGNs in the night
sky.
"Maybe there are some similar events that have already been observed—we just
don't know about them yet," Scepi said.
Reference:
ibasish Laha et al: "A radio, optical, UV and X-ray view of the enigmatic
changing look Active Galactic Nucleus 1ES 1927+654 from its pre- to
post-flare states", The Astrophysical Journal, 2022 May. arXiv:
arxiv.org/abs/2203.07446
Nicolas Scepi et al, Magnetic flux inversion in a peculiar changing look
AGN, Monthly Notices of the Royal Astronomical Society: Letters (2021).
DOI: 10.1093/mnrasl/slab002
Tags:
Space & Astrophysics